After reading the planetary assault and exterminatus scenarios and seeing all those nice asteroid fields lying around, I thought that why not use the asteroids for something creative besides just hiding... Dropping them on the biggest cities of the world in question! A certain B5 episode comes to mind;)
The mass drivers replace the ship's prow guns, and can only be taken by capital ships. Once the ship is in an asteroid field (this can be done while entering it) it can use reload ordnance to load the 'driver. While the ship is in a field and succeeds in reloading it may also choose to automatically burn retros to maintain its position.
An asteroid impacting on a world gives D6 (or D3?) assault points. If used in an exterminatus a ship doing the extermination gets +1 to its die roll for every impact since its last turn. If impacts cause more than 10 assault points, roll a D6 for every excess _point_ . The world might become uninhabitatet: 4+ for agri, 5+ for civilized and prison, 6+ for the rest.
Now, that was the easy part. I'm not sure how moving(and shooting at) the asteroid should be handled. I was thinking of handling the asteroid as ordnance, travelling 15 cm a turn. The same goes for shooting, except that it cannot be targeted by fighters, but bombers, each flight destroying the rock on a 6 (1 roll/flight). As for capital ships... I figure that an unpowered rock would be pretty hard to detect or to fire upon, especially if it didn't get hot in the firing process. Ld to target, always using the rightmost column, needing a 6 to hit? Lances hit as usual on a 6. Torpedoes hit on 6, but the asteroid gets to save on 4+ against them.
Ships may ram the asteroid, it counting as an escort-sized target. The impact does normal ramming damage to the rock (1 hit, armor 6), and normal asteroid-collision damage to the ship. Ld test required as normal. However, the come to new heading order is used instead of the all ahead full one.